Study of an asymmetric dual loop pulsating heat pipe: Visualization and parametric analysis

Abstract

This study explores the two-phase flow visualization and thermal performance of an asymmetric dual-loop pulsating heat pipe (adPHP) and an identical conventional dual-loop pulsating heat pipe (cdPHP) using a borosilicate glass tube setup. The asymmetric PHP, features a repetitive pattern of larger–smaller diameters (2.5 mm and 1.8 mm), incorporating diametrical asymmetry within the adiabatic section, while the conventional PHP maintains a uniform ID throughout. The asymmetric design aims to optimize two-phase flow start-up and maintain a unidirectional fluid circulation. The experimental setup was subjected to gradual heat-loading conditions. Observations revealed diverse flow behaviors at various locations within the PHP, influenced by factors such as inclination angles, filling ratios, working fluid and heat inputs. The research has investigated the impact of the initial fluid distribution within the PHP on the system’s start-up behavior, subsequently influencing the system’s pressure dynamics. The findings of this study indicate that the bubble nucleation and vapor plug expansion in the evaporator are two key mechanisms responsible for initiating the start-up process. The velocity head of the working fluid affects the condenser pressure, particularly during the start-up phase of the PHP. Alternate asymmetry promotes the start-up at low wall super-heat, and initial circulation inside the PHP at low heat loads. Enhanced thermal performance is observed with water as the working fluid at a 60% filling ratio, while acetone exhibits greater sensitivity to inclination angles, achieving optimal performance at a 40% filling ratio, indicative of a relatively less stable operational behavior.